1. Field of the Invention
The present invention relates generally to air conditioning systems for vehicles. In particular, the present invention relates to air conditioning systems with heat exchangers and motor-driven compressors.
2. Description of Related Art
Known vehicle air conditioning systems may determine a target rotational speed of a drive shaft of an operating compressor in response to an air conditioning load. Specifically, known air conditioning systems may control the rotational speed of the compressor, e.g., a drive shaft of the compressor, by controlling a rotational speed of an electric motor, e.g., a drive shaft of an electric motor, that drives the compressor. Further, known air conditioning systems may control the rotational speed of the electric motor based on the target rotational speed of the operating compressor. Moreover, known air conditioning systems may be operated for cooling or heating.
Nevertheless, in such air conditioning systems, if the electric motor starts to drive the compressor while a condensed, liquid refrigerant is present in a refrigerant pipe of a low pressure side of the air conditioning system, for example, if the air conditioning system is started when an ambient temperature outside is relatively low, e.g., at or below 0xc2x0 C., damage to the compressor may occur due to intake and compression of liquid refrigerant in the compressor. Moreover, the possibility of an occurrence of such damage may increase when the rotational speed of the drive shaft of the compressor at activation is relatively high.
In order to avoid the foregoing damage to the compressor, in the known air conditioning systems, particularly systems in which a high rotational speed of the compressor drive shaft is established as a target rotational speed with respect to an air conditioning load, the compressor is operated initially at a predetermined drive shaft rotational speed that is lower than the target rotational speed. Subsequently, the initial rotational speed of the compressor at the start of compressor operation is increased gradually toward the target rotational speed of the compressor.
Nevertheless, in such air conditioning systems, by starting the compressor at a drive shaft rotational speed that is lower than the target rotational speed of the compressor to avoid damage to the compressor, the compressor may not reach the target rotational speed as soon after the start of compressor operation as otherwise might occur if the compressor was not activated initially at a lower rotational speed to avoid damage to the compressor. Moreover, an increase in the time of operation of the compressor, e.g., a time of operation of the heating mode or the heating-dehumidifying mode, or the like, may result from the additional time required for the compressor drive shaft to reach the target rotational speed when the compressor is started initially at a drive shaft rotational speed that is lower than the target rotational speed. Further, even if the compressor is activated initially at or near the target rotational speed, the target rotational speed may be unnecessarily low, e.g., if the compressor is started in a heating mode of operation when the ambient, outside temperature is comparatively high, the rotational speed of the compressor may be limited unnecessarily.
A need has arisen for air conditioning systems for vehicles and methods of employing such air conditioning systems, wherein damage to a compressor, or to components of the air conditioning system, or both, due to compression of a liquid refrigerant may be reduced or eliminated. Moreover, a need has arisen for air conditioning systems for vehicles and methods of employing such air conditioning systems that reduce or eliminate such damage to a compressor while decreasing a time of operation of the compressor, e.g., a heating mode of operation, a heating-dehumidifying mode of operation, or the like, as compared to known air conditioning systems.
According to an embodiment of the present invention, a vehicle air conditioning system comprises a compressor, an electric motor for driving the compressor, means for variably controlling a rotational speed of a drive shaft of the compressor based on a target rotational speed of a drive shaft of the compressor; means for limiting the rotational speed of the drive shaft of the compressor to less than the target rotational speed when a first value of a current detected at the electric motor is greater than or equal to a first predetermined value; and means for terminating the limiting of the rotational speed of the drive shaft of the compressor when a second value of the detected current is less than a second predetermined value.
In a further embodiment of the present invention, a method of controlling an air conditioning system comprising a motor-driven compressor comprises the steps of detecting an electric current at an electric motor, comparing a first value of the detected current to a first predetermined value, and limiting a rotational speed of a drive shaft of the compressor if the first value is greater than or equal to the first predetermined value.
In yet another embodiment of the present invention, an air conditioning system comprises a compressor, an electric motor for driving the compressor, and a control device for regulating a rotational speed of the electric motor, so that a drive shaft of the compressor rotates at a target rotational speed based on an air conditioning load, wherein the control device detects a first value of an electric current at the electric motor and limits the rotational speed of the electric motor if the first value of the detected current is greater than or equal to a first predetermined value, so that the rotational speed of the drive shaft of the compressor remains below the target rotational speed.
Other objects, features, and advantages of embodiments of this invention will be apparent to persons of ordinary skill in the art from the following description of preferred embodiments with reference to the accompanying drawings.